1. Technical Field
The invention generally concerns enhancement filtering to improve visibility of blood vessels and more practically to a framework for vessel enhancement filtering in angiography images.
2. Description of the Related Art
The common way to interpret vasculature images, e.g. the Magnetic Resonance Angiography (MRA) images, is to display them in Maximum Intensity Projection (MIP) in which the stack of slices is collapsed into a single image for viewing. MIP is performed by assigning to each pixel in the projection the brightest pixel over all slices in the stack. With this type of display, small vessels with low contrast are hardly visible and other organs may be projected over the arteries. FIG. 1 may demonstrate that small vessels tend to resemble background. A vessel enhancement procedure as a pre-processing step for maximum intensity projection display will help to diminish these two limitations.
There are a variety of vessel enhancement methods in literature. The simplest one is to threshold the raw data but this makes the segmentation process incorrectly label bright noise regions as vessels and cannot recover small vessels which may not appear connected in the image. Recently, Hessian-based approaches have been utilized in numerous vessel enhancement filters. These filters are based on the principal curvatures, which are determined by the Hessian eigenvalues, to differentiate the line-like (vessel) from the blob-like (background) structures. However, their disadvantage is that they are highly sensitive to noise due to second-order derivatives. Moreover, they tend to suppress junctions which are characterized same as the blob-like structures using the principal curvature analysis. Junction suppression in turn leads to discontinuity of the vessel network.